Transcript Convection and Seafloor Spreading

Convection and Seafloor Spreading
 To fully understand the theories of Continental Drift and Plate
Tectonics, you must first have an understanding of the internal
processes within Earth.
 Convection currents in the asthenosphere, proposed by Arthur
Holmes, and the seafloor spreading idea, propose by Harry Hess,
provides evidence for a mobile Earth.
 By combining the seafloor spreading theory with
continental drift and earthquake information the new theory
of Plate Tectonics became a coherent theory to explain
crustal movements.
Reference:
527-532
Convection Currents
 The many plates that make up the earth’s crust sit directly on a
plastic like layer within the mantle called the Asthenosphere. A
scientist named Arthur Holmes provided evidence to prove that
tectonic plates moved on what he referred to as convection
currents.
 Convection can not take place without a source of heat.
Heat within Earth comes from two main sources;
1) radioactive decay, and 2) residual heat.
 If the asthenosphere is in fact moving as a result of
convection, then convection could be the mechanism
responsible for plate tectonics.
 Harry Hess influenced by Holmes’ ideas, suggested that
deep within the asthenosphere, heated material expands, becomes
less dense, rises, and pushes it way up through ridges. It then
moves along the base of oceanic plates, pulling the plates in
opposite directions. This concept we call Seafloor Spreading.
Convection Currents
 When this slowly moving material reaches cooler areas it contracts
and sinks causing one plate to move downward (subducting plate)
beneath another (over-riding plate). This material is then recycled
back into the mantle.
Ocean Depths
 During World War II, geologists employed by the military
carried out studies of the sea floor, a part of the Earth that
had received little scientific study.
 The topographic studies involved measuring the depth to
the sea floor. These studies revealed the presence of two
important topographic features of the ocean floor:
1) Oceanic Ridges - long sinuous ridges that occupy the
middle of the Atlantic Ocean and the eastern part of the
Pacific Ocean.
2) Oceanic Trenches - deep trenches along the margins of
continents, particularly surrounding the Pacific Ocean.
Ocean Depths
 Studies also noted that as oceanic lithosphere moves away
from the ridge, it cools and sinks deeper into the
asthenosphere. Thus, the depth to the sea floor increases
with increasing age away from the ridge.
Seafloor Sediment and Age
 Because the oceanic ridges are areas of young crust, there
is very little sediment accumulation on the ridges.
Sediment thickness increases in both directions away from
the ridge, and is thickest where the oceanic crust is the
oldest.
 As new oceanic crust is created it is pushed aside in two
directions. Thus, the age of the oceanic crust becomes
progressively greater in both directions away from the
ridge.
Younger
Older
Seafloor Sediment and Age
 Because oceanic lithosphere is created at rides and
destroyed at subduction zones (trenches), scientist noted
that the oceanic basins is continuously being recycled and
are relatively young. The oldest oceanic crust occurs
farthest away from a ridge. In the Atlantic Ocean, the oldest
oceanic crust is about 180 million years old (Jurassic in
age).
Sample Problem
Explain why it is impossible for oceanic crust to be older than
200 million years.
Answer:
A geologic process called sea floor spreading causes the
ocean floor to move. Ocean floor is created at oceanic ridges
and is destroyed or consumed within Earth at subduction
zones. This process of recycling the ocean floor occurs
within a span of 180 - 200 million years.